I am sorry and afraid but the order of the two photocopies are reversed and I don't know how to unscramble it.

The author means that these molecules are necessary to life and part of the chemical processes in living organisms.

What does 'what it is all about' mean?

What is it all about?

It is a general (sometimes rhetorical) question asking about the meaning and examples of "it."

eg;
What is life?
What is it (life) all about?

Does 'it' refers to 'life'?

contrapositionDoes 'it' refers to 'life'?

It refers to something in the prior context.

A: I remember the "War of the Worlds" radio broadcast and the panic it caused.
B: What was it all about? (it= the radio broadcast)

contrapositionWhat does 'what it is all about' mean?

I believe you are referring to the statement in the text "Oxygen is what it is all about".

This means that oxygen is the central and most important element within the discussion. Understanding this topic is mostly a matter of understanding the role of oxygen within these biological processes.